For level 15...hit the switch with the big guy, get on yellow platform after it moves left, jump down with big guy to activate button, press space to activate little guy, send little guy back up and over to where you originally were to activate two other buttons, blue cannon will fire and drop heart. And I agree with Anon 13:45 sloppy sloppy controls.

For level 15, you don't have to drop the big guy all the way down. Leave him up top. Use the little guy to flip the green (er, the first) switch - the cannon it activates knocks the big guy onto the pink switch. Hope that wasn't too confusing! :)